Often drill bits are subjected to harsh conditions when drilling below the earth's surface. Replacing damaged drill bits in the field is often costly and time consuming since the entire downhole tool string must typically be removed from the borehole before the drill bit can be reached. Bit whirl in hard formations may result in damage to the drill bit and reduce penetration rates. Further, loading too much weight on the drill bit when drilling through a hard formation may exceed the bit's capabilities and also result in damage. Too often, unexpected, hard formations are encountered suddenly and damage to the drill bit occurs before the weight on the drill bit may be adjusted.
The prior art has addressed bit whirl and weight on bit issues. Such issues have been addressed in the U.S. Pat. No. 6,443,249 to Beuershausen, which is herein incorporated by reference for all that it contains. The '249 patent discloses a PDC-equipped rotary drag bit especially suitable for directional drilling. Cutter chamfer size and back-rake angle, as well as cutter back-rake, may be varied along the bit profile between the center of the bit and the gauge to provide a less aggressive center and more aggressive outer region on the bit face, to enhance stability while maintaining side cutting capability, as well as providing a high rate of penetration under relatively high weight-on-bit.
U.S. Pat. No. 6,298,930 to Sinor, which is herein incorporated by reference for all that it contains, discloses a rotary drag bit including exterior features to control the depth of cut by cutters mounted thereon, so as to control the volume of formation material cut per bit rotation as well as the torque experienced by the bit and an associated bottomhole assembly. The exterior features preferably precede, taken in the direction of bit rotation, cutters with which they are associated, and provide sufficient bearing area so as to support the bit against the bottom of the borehole under weight-on-bit without exceeding the compressive strength of the formation rock.
U.S. Pat. No. 6,363,780 to Rey-Fabret, which is herein incorporated by reference for all that it contains, discloses a system and method for generating an alarm relative to effective longitudinal behavior of a drill bit fastened to the end of a tool string driven in rotation in a well by a driving device situated at the surface, using a physical model of the drilling process based on general mechanics equations. The following steps are carried out: the model is reduced so to retain only pertinent modes, at least two values Rf and Rwob are calculated, Rf being a function of the principal oscillation frequency of weight-on-hook WOH divided by the average instantaneous rotating speed at the surface, Rwob being a function of the standard deviation of the signal of the weight-on-bit WOB estimated by the reduced longitudinal model from measurement of the signal of the weight-on-hook WOH, divided by the average weight-on-bit defined from the weight of the string and the average weight-on-hook. Any danger from the longitudinal behavior of the drill bit is determined from the values of Rf and Rwob.
U.S. Pat. No. 5,806,611 to Van Den Steen, which is herein incorporated by reference for all that it contains, discloses a device for controlling weight-on-bit of a drilling assembly for drilling a borehole in an earth formation. The device includes a fluid passage for the drilling fluid flowing through the drilling assembly, and control means for controlling the flow resistance of drilling fluid in the passage in a manner that the flow resistance increases when the fluid pressure in the passage decreases and that the flow resistance decreases when the fluid pressure in the passage increases.
U.S. Pat. No. 5,864,058 to Chen, which is herein incorporated by reference for all that is contains, discloses a downhole sensor sub in the lower end of a drillstring, such sub having three orthogonally positioned accelerometers for measuring vibration of a drilling component. The lateral acceleration is measured along either the X or Y-axis and then analyzed in the frequency domain as to peak frequency and magnitude at such peak frequency. Backward whirling of the drilling component is indicated when the magnitude at the peak frequency exceeds a predetermined value. A low whirling frequency accompanied by a high acceleration magnitude based on empirically established values is associated with destructive vibration of the drilling component. One or more drilling parameters (weight-on-bit, rotary speed, etc.) is then altered to reduce or eliminate such destructive vibration.